(a) Field of the Invention
The present invention relates to a scroll-type expander and a scroll-type compressor, and more particularly, to a scroll-type expander and a scroll-type compressor that include a stationary scroll member and a rotating scroll member to continuously perform expansion and compression of an working fluid. The present invention also relates to a scroll-type heat exchange system that includes a scroll-type expander and scroll-type compressor for use as a Stirling engine or refrigerator.
(b) Description of the Related Art
Scroll device offers many advantages including high efficiency, low noise, low vibration, small size, and light weight. Scroll devices are widely used as a result of these advantages scroll-type compressor In more detail, with reference to FIG. 1, a stationary scroll member 30 of involute form and a rotating scroll member 40 are provided at a 180° phase difference. As a result, a series of crescent-shaped pockets are formed within the scroll-type compressor. Gas flows into the scroll-type compressor through an intake passage located at a circumference of the stationary scroll member 30, and the crescent-shaped pockets move toward a center of the two scrolls 30 and 40 by the orbiting action of the rotating scroll member 40. A volume of the pockets is reduced through this operation such that the gas is compressed. The gas is then discharged through a discharge port formed in a center of the stationary scroll member 30. During each orbit, several crescent-shaped pockets are compressed simultaneously, so operation is continuous.
In the scroll-type expander, the scroll-type compressor is simply operated in reverse such that a gas is expanded. That is, a high pressure gas is provided to the center of the stationary scroll member 30 such that the orbiting scroll member 40 is displaced to realize expansion of the gas, which is then discharged through the circumferential opening of the stationary scroll member 30. Motive power is generated by the orbiting motion of the rotating scroll member 40.
Compared to other types of compressors, the scroll-type compressor requires less parts, is small and lightweight, and provides other advantages such as high efficiency, low vibration, and low noise. As a result, the scroll-type compressor is widely used as a refrigerant compressor and air compressor. The scroll-type expander, on the other hand, has not experienced widespread use.
As a conventional expander, U.S. Pat. No. 4,192,152 discloses a scroll apparatus with peripheral drive that can be used as a compressor and an expander, and a heat engine that combines a compressor, a burner, and an expander, and also discloses a Brayton cycle-type cooling cycle that combines a compressor and an expander. Also, EP Patent No. 0846843A1 discloses a heat engine that combines a compressor, a regenerator, a burner, and an expander. In addition, there has also been recently disclosed in the United States a steam cycle (Rankine system) that uses a scroll-type expander in place of a steam engine.
However, in patents and research related to scroll-type expanders disclosed up to now, high pressure gas or steam is supplied to a center area of the scroll-type expander to generate motive power as in conventional turbines. As a result, efficiency is reduced by pressure loss when supplying the gas or steam such that while compression efficiency reaches up to 90%, expansion efficiency is only about 60˜70%. Further, in the conventional scroll-type expander, a difference in temperatures between the stationary scroll member and the orbiting scroll member develops, and a temperature gradient occurs within the same scroll wrap itself. These factors result in a reduction in efficiency by the generated friction, leakage, and increased vibration.
A Stirling engine is an external combustion engine that includes a plurality of heat exchangers that heat and cool the enclosed charge gas. Most Stirling engines are external combustion engines of reciprocating piston types.
Because the Stirling engine is an external combustion engine, it may use various heat sources such as liquid fuel, gas fuel, solid fuel, industrial waste energy, solar energy, and LNG. The Stirling engine provides high efficiency due to a regenerator mounted between a heater and a cooler. Also, because the Stirling engine does not include valves and realizes smooth pressure changes, a low level of noise and vibration are generated compared to the internal combustion engine. Also, since continuous combustion occurs in the Stirling engine, combustion control is easy and the exhaust gas is relatively clean, thereby making the Stirling engine a possible candidate for widespread use in the future.
With reference to FIG. 8, which shows a basic structure of a conventional Stirling engine 200, an expansion piston 201 and a compression piston 203 are coupled to a common crankshaft with about 90° phase difference. An expansion space 205 and a compression space 207 are formed and connected to a regenerator 209 that is filled with thermal energy storage material having gas permeability. With this configuration, since it is difficult to realize sufficient heating and cooling of the working gas by a cylinder wall of a small heat transfer area, a cooler 212 and a heater 214 are provided to opposite sides of the regenerator 209 as shown in FIG. 9.
To simplify the mechanical structure and reduce vibration of the reciprocating-I Stirling engine, U.S. Pat. No. 6,109,040 discloses a configuration that uses two rotary Wankel rotors and provides for a phase difference as in the reciprocating Stirling engine such that compression and expansion are alternatingly realized.
Since two pistons reciprocate in cylinders synchronously but out of phase so that the working gas shuttles cyclically from one space to the other as the volume and pressure vary from maximum to minimum and go through the four processes of the Stirling cycle in order, the working fluid undergoes pressure loss due to the oscillating flow through the regenerator positioned between the compression cylinder and the expansion cylinder such that an increase in rotational speed results in the reduction in torque. In addition, because it is difficult to realize sufficient heating and cooling of the working gas by a cylinder wall of a small heat transfer area, the cooler 212 and the heater 214 are provided to opposite sides of the regenerator 209 as shown in FIG. 9, and it is necessary to use a gas having a low molecular weight such as hydrogen or helium as the working gas. However, in the case where a gas of a low molecular weight is used as the working gas, leakage easily occurs such that it is extremely important to use a high performance gas seal.
With reference to FIGS. 10 and 11, an ideal Stirling cycle includes isothermal compression (I–II) while in a low temperature compression section 223, constant volume heating (II–III) while passing a regenerator 221, isothermal expansion (III–IV) while in a high temperature expansion section 224, and constant volume heat rejection (IV–I) while passing the regenerator 221. However, the actual cycle is more like that shown in FIG. 12, which is significantly less efficient than the ideal case. The reasons for such a difference between an ideal Stirling cycle and the actual cycle, and the difficulties in realizing the ideal cycle, will be described as follows.
First, to realize the isothermal compression (I–II) and isothermal expansion (III–IV) sections of the ideal Stirling cycle, fast heat transfer must occur through the inside surface of the cylinder walls. However, even if a sufficient number of heat transfer pins are mounted outside the cylinder, since the area of the inside surface of the cylinder walls making contact with the working gas is limited, it is difficult for the working gas to be heated or cooled isothermally. This becomes increasingly problematic if the engine is made faster and to larger sizes, in which case the processes inside the cylinder becomes more adiabatic (no heat transfer) than isothermal (infinite heat transfer).
It is for this reason that the additional heater 214 and cooler 212 are mounted to opposite ends of the regenerator 209 to ensure effective heating and cooling of the working gas. Although the heater 214 and cooler 212 allow for the effective heating and cooling of the working gas to increase the specific power, the provision of such heat exchangers imposes some penalties as follows.
In particular, the increase in dead volume, which includes the heater 214, the regenerator 209, and the cooler 212, acts to decrease output. Further, it results in anomalies in which the expanded working gas picks up heat from the heater 214 before deposing its heat in the regenerator 209 and in which the compressed gas has to pass through the cooler 212 before going back through regenerator 209 to pick up heat. As a result, the flow resistance is increased and thermal efficiency is reduced. Further, the thermal stress to the structural parts increases such that care must be given in selecting the materials for the parts and other limitations are given to manufacture of the device.
In the ideal Stirling cycle as shown in FIG. 10, since the motions of the pistons 225 and 227 are discontinuous, only compression occurs in the low temperature compression section 223 and only expansion occurs in the high temperature expansion section 224. However, in an actual reciprocating Stirling engine shown in FIG. 9, the compression piston 203 and the expansion piston 201 are linked to move together such that during compression by the compression piston 203 of the low temperature section, compression occurs slightly also by operation of the expansion piston 201 of the high temperature section. Likewise, during expansion by the expansion piston 201 of the high temperature section, expansion occurs slightly also by operation of the compression piston 203 of the low temperature section. This is another main reason why the efficiency of the actual Stirling engine is significantly less than that of the ideal Carnot engine.
The steam cycle includes four successive changes. These include heating of the working fluid, evaporation, expansion, and condensation. The Rankine cycle is the ideal cyclical sequence of changes of pressure and temperature of the working I fluid, and is used as a standard for rating the performance of steam power plants.
With reference to FIG. 13, a steam engine 300 typically includes a water supply pump 303 (adiabatic compression), a boiler 305 and a re-heater 307 (isobaric heating), turbines 309 and 312 (adiabatic expansion), and a condenser 301 (isobaric heat radiation). A steam turbine is most commonly used by a power output device in the steam engine that is used as an external combustion engine. The steam turbine converts heat energy into kinetic energy such that high speed steam strikes a turbine to obtain a rotational force of the same.
As a way to improve efficiency in the steam cycle, referring again to FIG. 13, the re-heater 307 is used and the steam in the expansion stage is extracted to the outside of the turbine 309 before being saturated, and is made into superheated steam after being heated in the re-heater 307. The steam is again directed to the turbine 312 to use a re-heating cycle that expands the steam until reaching the output pressure. Thermal efficiency may be improved by increasing the number of re-heating stages. However, if the number of re-heating stages is increased, the fluid needs to be circulated between the boiler 305 and turbines 309 and 312, both the overall size of the assembly and equipment costs are increased, and operational control becomes complicated. Accordingly, re-heating is typically performed one or two times, which places a limitation on the efficiency of the steam cycle.
In the reciprocating piston or Wankel rotary device, which are conventional positive displacement expanders used as external combustion engines in place of the steam turbines 309 and 312, since the area of heat transfer through the cylinder walls decreases compared to volume as capacity is increased, efficiency reduces in proportion to increases in size of the device.